Thermotropic liquid-crystalline polyester resin (hereinafter, called liquid-crystalline polyester resin or LCP) exhibits good flowability on molding and excellent thermostability, and therefore, is widely used for manufacturing molded articles, especially electronic components having thin and small parts such as fine pitch connector. LCP also has good mechanical properties including good rigidity, excellent chemical resistance, good gas barrier properties and exhibits high dimensional accuracy. Due to those properties, LCPs are used not only for manufacturing molded articles but also for manufacturing a variety of products including fibers and films.
Among LCPs, there are semi-aromatic liquid-crystalline polyester resins and wholly aromatic liquid-crystalline polyester resins. The former has aliphatic moieties such as ethylene group and 1,4-butylene group in the molecular chain, and the latter has little or no aliphatic moieties in the molecular chain. Among them, wholly aromatic polyester resins are preferably used because of their heat resistance and good mechanical properties.
Though liquid-crystalline polyester resin has excellent properties as described above, the resin may sometimes have only insufficient mechanical properties and color tones due to high temperatures upon the polycondensing or molding step.
In order to improve mechanical properties of the liquid-crystalline polyester resin, WO01/81449 and JP 2003-160716 disclose a method for manufacturing LCP comprising copolymerizing major monomer components and low amount of minor copolymerizing component in the presence of a certain amount of alkali metal compound, wherein the minor copolymerizing component is selected from the group consisting of 4-hydroxyisophthalic acid, salicylic acid, 3-hydroxy-2-naphthoic acid and 2-hydroxynaphthalene-3,6-dicarboxylic acid. However, the method has a problem that color tone of the resin is difficult to be controlled and the obtained LCP is unsuitable for the use where coloring agents such as carbon black are not used for coloring the resin.
Because of these problems, methods which does not involve impairment of mechanical properties or color tone due to high polycondensation temperatures and thermal degradation upon molding are desired.
For example, Japanese Patent Application Laid Open No. 10-316873 discloses a method for manufacturing LCP comprising admixing a benzoxazole compound into resins in molten phase. Japanese Patent Application Laid Open No. 2-51524 discloses a method for manufacturing LCP comprising admixing organic additives such as organic phosphorous compound and hindered phenol compound into resin before polycondensation reaction is completed. In addition, Japanese Patent Application Laid Open No. H11-199761 discloses a method for manufacturing LCP which comprises admixing salts such as primary phosphate (dihydrogenphosphate) into liquid-crystalline polyester resin by the use of extruder.
With regard to semi-aromatic liquid-crystalline polyester resin, Japanese Patent Application Laid Open No. H8-53605 discloses a method comprising adding phosphorous acid or hypophosphorous acid and the metal salts thereof into resins and Japanese Patent Application Laid Open No. H8-165411 discloses a method comprising admixing non-fibrous salts such as sodium dihydrogen phosphate into resins before polycondensation is completed.
However, organic additives such as benzoxazole compound, organic phosphorous compound and hindered phenol compound can not exert sufficient effect on improving color tone because of their low thermostability and volatility. In addition, by the method comprising admixing primary phosphate (dihydrogenphosphate) into resins by extruder, LCP with good color tone and thermostability can not be stably obtained because the coloring of resins upon polycondensation can not be suppressed and small-amount of salts can not be evenly dispersed.
The method disclosed in Japanese Patent Application Laid Open No. H8-53605 which is directed to semi-aromatic liquid-crystalline-polyester resin can not be applied to a wholly aromatic liquid-crystalline polyester resin because the improvement of color tone can not be attained when the method is applied to wholly aromatic LCPs.
The method disclosed in Japanese Patent Application Laid Open No. H8-165411 has similar problems with that of Japanese Patent Application Laid Open No. H8-53605. Japanese Patent Application Laid Open No. H8-165411 also discloses a method for manufacturing a semi-aromatic liquid-crystalline polyester resin comprising the step of acylating monomers and polycondensing the monomers in the presence of non-fibrous salts such as sodium dihydrogen phosphate. However, the resulting semi-aromatic liquid-crystalline polyester resin may exhibit only poor mechanical properties such as flexural strength.